(1) Field of the Invention
The present invention relates to an image forming apparatus performing both a print function and a scan function, in particular to a technology to correct tone properly. Here, the print function includes forming an image on an, image forming sheet, and the scan function includes reading the image of an original and generating image data.
(2) Description of the Related Art
Conventionally, image forming apparatuses perform an image stabilization process such as a registration correction process or a tone correction process in order to achieve high-quality images. For example, an image forming apparatus (multifunction machine) including a print function and a scan function prints a color shift detection pattern, a tone correction pattern or the like on a sheet of paper, reads the image by scanning the printed paper, and corrects a color shift, tone, and the like in the print function, while taking into consideration uneven rotational periods of an image carrier (Japanese Laid-Open Patent Application Publication No. H10-126574, Japanese Patent Publication No. 3648131, Japanese Laid-Open Patent Application Publication No. 2006-349851).
However, according to the above-mentioned conventional methods, an area of each tone patch in the tone correction pattern needs to be larger than a certain degree, as an inconsistent speed of paper conveyance while printing and scanning the tone correction pattern, a positional shift due to vibration of the apparatus, uneven print and the like need to be taken into account. For example, a conventional tone correction pattern of a four-color image forming apparatus includes a double line of 16 tone patches for each color, which is printed in the main scanning direction of a sheet of A4-sized recording paper. In this way, each recording paper can test 32 tone patches for each color. Accordingly, in order to perform a tone correction with high accuracy, several pages of printing and scanning are required, which is time-consuming and thus undesirable. Also, if the tone correction is to be performed in a relatively short period of time, the number of patterns for the correction will be limited, affecting the accuracy of the tone correction.
Additionally, here, if a toner marker or the like indicating a position of a tone correction pattern is printed at the starting position of the tone correction pattern to improve the positional accuracy, it will not be effective to an overall magnification error and a local magnification error which is caused by an inconsistent speed, vibration of the apparatus, and the like.
In addition, since color shift correction patterns need to be printed as well, a display area on the recording paper will be further limited.
In general, in view of user's convenience, it is preferable that a time required to perform an image stabilization process be short. Thus, it is preferable that tone correction patterns and color shift correction patterns be kept in a sheet of recording paper of a common size such as A4.